Neurons in lamina I and II of the lumbar spinal cord of the cat were characterized physiologically in terms of responsiveness to various natural stimuli (such as pinch and brush), peripheral input (C fiber vs. A-delta vs. A-beta), and effects of nucleus raphe magnus stimulation. These neurons were then intracellulary injected with horseradish peroxidase, the cats perfused, and the spinal cord sectioned and reacted with diaminobenzadine. The nerons were identified, the tissue processed immunohistochemically with antibodies to serotonin and substance P, and the cells and immunoreactive boutons then drawn by camera lucida technique with high power light microscopy